| The bidirectional high-frequency isolated DC-DC converter has been widely applied in the energy storage system for microgrid because of its advantages of high-frequency electrical isolation,high power density and high efficiency.In order to solve the problems of the existing technologies,such as the high current stress,low utilization rate of hardware capacity and poor economy,a novel bi-directional buck-boost current-fed isolated DC-DC converter is proposed.It has the lowest current stress of the power devices in theory,and has significant advantages in terms of the cost and efficiency.In this paper,the topology,operational mechanism,modulation strategy,mathematical model and closed-loop control strategy of the proposed converter are researched in detail.Firstly,the topology,operational mechanism and modulation strategy of the novel converter are researched.The relationship between the averaged inductor current and the duty cycle of half bridge in the input side and the short-through duty cyle of the full-bridge inverter is obtained.On this basis,a no-voltage-spike cooperative modulation strategy is proposed to ensure the reliable operation of the converter.Furthermore,a dual-mode control scheme is proposed to realize the minimum averaged inductor current in the entire operation range,so as to improve the efficiency of the converter.Taking the minimum inductor current ripple as the optimization objective,the optimal phase of the conductive range of the input-side switch is obtained.Secondly,the switching process of the two operation modes of the novel converter is analyzed,and it is revealed that the directly switching between the two operation modes will lead to the transient voltage spike and large fluctuations of the inductor current.In order to solve this problem,taking the inductor current constant during the mode switching process as the constraint,a smooth switching strategy of the operation modes is proposed.Furthermore,in order to ensure the safe operating of the converter,the mechanism of the transient voltage spike caused when the short through duty cycle is reduced unconstrainedly is analyzed in detail.The method of eliminating the transient voltage spike based on constraining the reduction rate of duty cycle is proposed and the calculation basis of the allowed maximum reduction rate is accordingly obtained.Then,in order to obtain the high control performance of the proposed converter,the linear mathematical model is biult and the closed-loop control strategy of the converter is researched.The linear models of the buck mode and the boost mode of the converter are originally derived using the state-space averaging method.The dual-closed-loop control configuration with the inductor current and output voltage is designed.In order to ensure the stability of the both operation modes with the same control parameters,on the basis of the linear models,the parameters of the controllers are designed using the method of Hurwitz criterion plus frequency response.The asymptotic stability of the operation conditions with periodic switching between the two modes caused by the periodic fluctuation of input voltage is verified using the linear switching theory.Finally,taking DSP and FPGA as the control cores,the experimental platform of the proposed converter is built.Its hardware and software are designed in detail.The proposed converter topology,all the theoretical analysis results,the corresponding modulation and control strategies are verified in detail.At last,the proposed converter is compared with some existing bi-directional isolation converters,which highlights the advantages of the proposed converter in terms of current stress of the power switches and cost. |
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